It is well known that when a tire fitted on a vehicle rolls on a surface, the tire gives rise to an audible sound, which may be unpleasant both for the driver of the vehicle and for the persons in the vicinity of the vehicle. Therefore, tire manufacturers have been trying to reduce the noise emission of tires for a long time.
A great variety of techniques have been proposed. For example, it has been proposed to provide noise absorbers within the tire or the wheel to which the tire is fitted (see, for instance, documents US 2006/0289100 and US 2008/0116612). There have also been numerous attempts to reduce tire noise by adapting the tread pattern, for example via the “variable pitch” technique (see, for instance, document U.S. Pat. No. 4,598,748).
One of the significant contributions to the noise produced by a tire is due to the excitation of the air contained inside the tire cavity: there is an excitation by the road and by the deflection of the tire tread and sidewall when the tire is rolling on a road. The effects caused by acoustic resonance of the air contained in a tire, such as the “first cavity mode” (FCM) have been studied in detail. Document WO 2008/071422, to cite only one example, describes a way to reduce this noise component by providing a resonating cavity in the bead, the cavity being configured to be in fluid communication with the tire cavity.
Notwithstanding all these efforts, there is still a great need for reduction of tire noise, in order to satisfy the ever-increasing demands of car manufacturers and lawmakers.
One promising field of research for noise reduction consists in the study of the vibratory behaviour of the tire itself. It is known (see, for example, the monograph “The tire. Mechanical and acoustical comfort”, published by Michelin in 2002, chapter III.3) that the vibratory behaviour of a tire varies according to frequency. Below 30 Hz, the tire acts like a spring. Between 30 Hz and 250 Hz, the tire may be considered to be a multi-mode vibratory system as it has several natural mode shapes, all of which may be grouped into two main categories: radial modes and transversal modes. At frequencies above 250 Hz, the tire mainly vibrates near the contact patch.
The applicant has studied the vibratory behaviour of tires and found that the tire meridian modes have a considerable impact on the noise generated by the tire. “Meridian modes” are understood to designate vibratory modes wherein the tire is deformed in a direction normal to its meridian such as shown in FIG. 6. (For a thorough treatment of vibratory modes of a tire, see also “The identification of sound generating mechanisms of tires” by Byoung Sam Kima, Gi Jeon Kimb and Tae Keun Lee in Applied Acoustics, 68/1 (2007), 114-133.)
Meridian modes are also sometimes referred to as “flexural modes” and classified according to the number of anti-nodes (i.e. points which undergo vibrations between a large positive and large negative displacement, as opposed to nodes, which are points that appear to be standing still) along the tire in a radial section. It has been found that the flexural mode at 5th order (i.e. the flexural mode with five anti-nodes in a meridian plane, such as the mode depicted in FIG. 6) are important contributors to tire noise.